ผู้เขียน: Snezhana Hristova

Ravi P. Agarwal is a professor and the chair in the Department of Mathematics at Texas A&M University, Kingsville. Snezhana Hristova is a professor in the Department of Applied Mathematics and Modeling at Plovdiv University in Plovdiv, Bulgaria. Donal O”Regan is a professor in the School of Mathematics, Statistics and Applied Mathematics at the National University of Ireland in Galway, Ireland.